1. Field of the Invention
The present invention relates to an inkjet printing apparatus and inkjet printing method which print by discharging ink from a printhead onto a printing medium.
2. Description of the Related Art
There are various kinds of printing apparatuses such as image print apparatus of, e.g., a printer, copying machine, and facsimile, a multifunction electronic apparatus including, e.g., a computer and word processor, and a print output apparatus of, e.g., a workstation. These printing apparatuses print images and the like on printing media such as printing paper and a thin plastic plate based on image information (containing all output information such as text information).
Such printing apparatuses can be classified into, e.g., the inkjet scheme, wire dot scheme, thermal scheme, and laser beam scheme in accordance with their printing methods. A printing apparatus (to be referred to as an inkjet printing apparatus hereinafter) of the inkjet scheme prints by discharging ink from a printhead onto a printing medium. The inkjet printing apparatus has various advantages of easy high-precision printing, high-speed printing, excellent quietness, and low cost as compared with the other printing schemes. Along with the recent increase in the importance of a color output such as a color image, a variety of color inkjet printing apparatuses which attain high quality comparable even to that of a silver halide photograph are under development.
To improve the printing speed, a general inkjet printing apparatus of this type uses a plurality of printheads (multiheads) which are formed by integrating a plurality of printing elements including, for example, ink discharge orifices and ink channels and are compatible with color printing.
FIG. 1 shows the arrangement of an inkjet printing apparatus which prints using the above-described multiheads. Referring to FIG. 1, ink cartridges 101 include printheads 102 serving as multiheads and ink tanks containing inks of four colors, black, cyan, magenta, and yellow. FIG. 2 shows ink discharge orifices arrayed on the printhead 102 when seen from the Z direction. n ink discharge orifices 201 which constitute a printing element are arrayed on the printhead 102 with a density of N dots per inch (N dpi). Referring back to FIG. 1, a conveyance roller 103 rotates in a direction indicated by an arrow in FIG. 1 while holding down a printing medium P together with an auxiliary roller 104, thereby conveying the printing medium P in the Y direction as needed. A feeding roller 105 feeds a printing medium P and also serves to hold down the printing medium P, like the conveyance roller 103 and auxiliary roller 104. A carriage 106 supports the four ink cartridges 101 and moves them as printing progresses. When, for example, printing is not performed or the printhead 102 undergoes a recovery operation, the carriage 106 stands by at a home position h indicated by a dotted line in FIG. 1.
Upon receiving a printing start instruction, the carriage 106 which has been at the home position h before the start of printing moves in the X direction. During this movement, the n ink discharge orifices 201 arrayed on the printhead 102 with N dpi print an image pattern with a width of n/N inches on a printing medium P. After the printing of the trailing edge of the printing medium P is completed, the carriage 106 returns to the original home position h and performs printing scanning in the x direction again. Before the start of the second printing after the completion of the first printing, the conveyance roller 103 rotates in the direction indicated by the arrow to convey the printing medium P in the Y direction by a width of n/N inches. For each scanning of the carriage 106, the printing of an image pattern with a width of n/N inches by the printhead 102 and the conveyance by the same width are repeated. This makes it possible to complete the printing of an image corresponding to, for example, one page. Such a printing mode in which an image is printed by performing printing scanning in the same printing region once is called a one-pass printing mode.
The one-pass printing mode is suitable for high-speed image printing. However, a few small errors are sometimes occurred in this mode generally due to a conveyance operation by a conveyance mechanism. FIGS. 3A to 3C each illustrate a printing example in which an error (conveyance error) is occurred due to the conveyance operation. FIG. 3A illustrates a case in which the conveyance is performed ideally. FIG. 3B illustrates a case in which a gap is formed with a width S because the contact portion between dots printed by the Kth scanning and (K+1)th scanning is discontinuous. If a gap with a width S is occurred due to a conveyance error as in this case, an unprinted stripe with a width S appears in the scanning direction of the printhead, resulting in a decrease in the quality of a printed image. As an example of a measure against this problem, Japanese Patent Laid-open No. S61-121658 discloses a method of printing by matching image regions in the contact portion between successive scanning operations and complementing the matched image regions with each other by these scanning operations, as shown in FIG. 3C.
The quality of a printed image decreases due to an unprinted stripe occurred in the contact portion not only when a conveyance error is occurred but also when ink droplets discharged from the printhead do not scatter straightly. U.S. Pat. No. 6,375,307 discloses an example of a measure against a decrease in the quality of a printed image as in this case.
High-quality image printing involves various factors such as the color development, tonality, and uniformity. In particular, the uniformity readily decreases when a slightest manufacturing variation unique to each nozzle occurs in a multihead manufacturing process. This variation adversely affects the discharge amount and discharge direction of ink from each nozzle in printing and finally causes density unevenness of a printed image.
A detailed example of this phenomenon will be explained with reference to FIGS. 4A to 4C and 5A to 5C. Referring to FIG. 4A, a printhead 102 includes eight ink discharge orifices 201. Ideally, ink droplets 43 are normally discharged from the ink discharge orifices 201 by the same amount and in the same direction, as shown in FIG. 4A. Discharge in this way forms dots with the same size in a uniform array pattern on a printing medium, as shown in FIG. 4B. A uniform image free from any density unevenness as a whole is thus obtained, as shown in FIG. 4c. 
However, individual nozzles actually have manufacturing variations as described above. When printing is performed in the one-pass printing mode, the sizes and discharge directions of ink droplets discharged from the ink discharge orifices vary, as shown in FIG. 5A. These ink droplets land on a printing medium, as shown in FIG. 5B. Referring to FIG. 5B, unprinted portions and, conversely, excessively superimposed dots extend in the scanning direction (the horizontal direction in FIG. 5B) of the printhead. An unprinted stripe is also occurred around the center in FIG. 5B. Portions printed in this state have a density distribution as shown in FIG. 5C in the array direction of the ink discharge orifices, and therefore are detected as density unevennesses. A stripe (contact stripe) formed in the contact portion between successive scanning operations often becomes conspicuous due to a variation in the amount of conveyance.
As a measure against these density unevenness and contact stripe, Japanese Patent Laid-open No. S60-107975 discloses the following method for a monochrome inkjet printing apparatus. This method will be briefly explained with reference to FIGS. 5A to 5C and 6A to 6C. This method scans the printhead 102 three times to complete the printing of printing regions shown in FIGS. 5B and 6B (FIG. 6A). The printing of a four-pixel region corresponding to 1/2 each printing region is completed by two-pass printing. In this case, the eight nozzles of the printhead 102 are divided into two groups, that is, four upper nozzles and four lower nozzles in FIG. 5A. Dots printed by the first scanning using each nozzle are thus thinned out to about 1/2. The remaining half dots complementary to the dots printed by the first scanning are printed by the second scanning to complete the printing of a four-pixel region. The above-described printing mode will be referred to as a multipass printing mode hereinafter.
The use of this multipass printing mode allows reduction of the adverse influence of a manufacturing variation unique to each nozzle on a printed image by half even when the printhead shown in FIG. 5A is used. A printed image as shown in FIG. 6B is thus obtained. In this image, an unprinted stripe and overprinted stripe (stripes occurred upon superimposition of dots) as shown in FIG. 5B are less conspicuous. A uniform density distribution as shown in FIG. 6C is thus obtained. In this density distribution, density unevenness is considerably small as compared with that caused in the one-pass printing mode. In this multipass printing mode, image data is divided and printed so that the image data printed by the first scanning and second scanning complement each other in accordance with a predetermined array pattern. The most common mask pattern used to divide this image data is the one which prints a staggered pattern in the vertical and horizontal directions pixel by pixel, as shown in FIGS. 7A to 7C. The printing of a unit printing region (a four-pixel region in this case) is completed by the first scanning for printing a staggered pattern and by the second scanning for printing a pattern complementary to that printed by the first scanning. FIGS. 7A to 7C explain how to complete the printing of a predetermined region when a mask pattern printed in this way is used by taking a case in which a multihead having eight nozzles is used as in FIGS. 4A to 6C as an example.
First, in the first scanning, a staggered pattern is printed on a printing medium using the four lower nozzles shown in FIG. 5A (FIG. 7A). Next, in the second scanning, the printing medium is conveyed by four pixels (1/2 the length of the printhead), and image data complementary to that printed by the first scanning is printed (FIG. 7B). Lastly, in the third scanning, the printing medium is further conveyed by four pixels again, and printed in the same manner as in the first scanning (FIG. 7C). The conveyance by four pixels and the printing of complementary staggered patterns are alternately repeated in this way, thereby completing the printing of a four-pixel region for each scanning. As described above, when the printing of the same printing region is completed using two different nozzles, it is possible to obtain a high-quality image free from any density unevenness.
Unfortunately, the conventional inkjet printing scheme poses the following problems. To obtain a high-quality image at high speed, it is necessary to discharge small liquid droplets with high frequency. This occurs a stripe as in the printing result shown in FIG. 8. A stripe of this type is particularly occurred in a region with high dot density (high printing duty), such as the contact portion between successive scanning operations of the printhead.
The cause of this phenomenon will be explained with reference to FIG. 9. FIG. 9 is a view showing the state in which the printhead 102 discharges ink droplets in printing the printing result shown in FIG. 8. FIG. 9 shows the state in which all of a plurality of nozzles (e.g., 256 nozzles) of a printhead discharge ink droplets, that is, the state in which printing is performed with a printing duty of 100%. Ink droplets discharged from nozzles in the edge portions of the nozzle array scatter inward with respect to the nozzle array. This is because all the nozzles discharge ink with high frequency and the air surrounding the discharged ink droplets migrates in the same direction, so the air pressure is reduced. This produces an air current in which the air outside the reduced pressure portion migrates toward it, and therefore the ink droplets discharged from the nozzles in the edge portions curve inward. In this specification, this phenomenon will be referred to as edge deviation hereinafter. When this edge deviation occurs, the landing positions of dots formed by the ink droplets discharged from the nozzles in the edge portions of the nozzle array shift, resulting in a stripe as in the printing result shown in FIG. 8.
To avoid this edge deviation, the volumes of discharged ink droplets may be increased. This makes it possible to suppress the adverse influence of an air current produced under a reduced pressure on a printed image. However, as the volumes of discharged ink droplets increase, ink dots become conspicuous in a printed image, resulting in degradation in image quality. Although edge deviation can be reduced by decreasing the discharge frequency, the number of nozzles, or the density of nozzles, the printing speed drops. Still worse, change in the printhead arrangement may increase the manufacturing cost.
This edge deviation depends on the density (printing duty) of dots printed by one scanning operation. For this reason, edge deviation occurs not only in printing in the one-pass printing mode as shown in FIG. 8 but also in printing in the multipass printing mode.